As a wireless communication system becomes more complicated, a linearity of a power amplifier is more critical, and a spectral re-growth for compensating the power amplifier distortion has more challenge. FIG. 1 shows a nonlinear input capacitance of the power amplifier due to the AM-PM distortion. As shown in FIG. 1, when the input power increases, the input capacitance of the power amplifier may suddenly drop, and the efficiency of the power amplifier may be worsened.